We propose to address gaps in present knowledge on girls with Attention Deficit Hyperactivity Disorder (ADHD). Only two studies have examined the effectiveness of stimulants for ADHD girls, and both acknowledged limitations and urged replication with larger samples and more extensive assessment. Comparable cognitive deficits for ADHD boys and girls have been reported often, but studies have not examined specific aspects of attention, as opposed to energetic processes. We will test 30 ADHD girls, 30 ADHD boys, 30 normal girls, and 30 normal boys. Drug-free ADHD children will be contrasted to comparable like-sex controls on timing and activation of stimulus evaluation and response preparation in a selective attention (Eriksen flanker) task. Performance and event-related potentials (ERP; P3b and lateralized readiness potential) in these tasks will cast light on these issues. ADHD children will enter a double-blind crossover trial consisting of 3 consecutive weeks each of methyphenidate (0.75 mg/kg/day) and placebo. Teacher and parent ratings, arithmetic test, observations in a restricted academic situation, paired-associate learning test, and cognitive ERPs and performance in the flanker test will be studied during each phase of the trial. Controls will be tested at comparable intervals. We will assess the extent of sex differences in (a) improvement with stimulants, and (b) the degree of normalization obtained by comparing patients to controls at similar test points. The results should provide information on sex differences in ADHD cognitive deficits and their modification by stimulants in a statistically adequate sample.